They really don't do much without the counter parts...ie: headers and a cat-back.
But they do add a little quickness though...but the headers will add more, and then the cat-back will make it all come together
and you can really feel it then.

How about an intake? Cheapest bolt on for the horsopower. Then maybe nitrous is the cheapest mod to increase horsepower. I would get an NX kit with a remote bottle opener and AEM's fuel rail so that you can hook up the NX kit easily. Remember that with the increased horsepower it's good to upgrade your brakes. I would at least bleed them and replace the brake fluid with some good DOT4 fluid. Then replace the pads with some AEM/Nissin ones. Then maybe the rotors. Some cross-drilled and slotted Brembo rotors would be nice. There's a group buy going on right now for them.

You should be able to install the intake and header yourself, but i wouldn't try to install the suspension parts becauae if you don't know what you're doing you can really screw something up. Just my .02...